The present invention relates to semiconductor devices and, more particularly to semiconductor switches suitable for use in trimming circuits, especially for frequency trimming of an internal clock source such as a relaxation oscillator.
One known relaxation oscillator circuit is described in U.S. Pat. No. 7,005,933. The electrical current, which is passed through a capacitor, creates a capacitor voltage, which is a ramp voltage. A comparator compares a known reference voltage with the ramp voltage and when the two are equal, the comparator switches, creating a clock signal. For effective operation of the oscillator, the circuit must have capacitors of the proper specification for creating the capacitor voltage.
Capacitors often vary in their characteristics due to the fabrication process. This variation can be compensated for by employing digitally-controlled trimmable components. These trimmable components are current sources operating together to produce a summed current, which is used to control the value of a total current that is fed to the capacitor for generating the capacitor voltage and thereby compensate for any process variation in the capacitors.
One example of a trimming circuit is described in U.S. Patent Publication No. 2009/0072804. This trimming circuit supplies a trimmed voltage. The trimmed voltage is generated from an external voltage by selectively switching in or out one or more of a plurality of resistors using a plurality of switches that are formed of P-N paired transfer gates. The switches are controlled by a decoder.
Another example of a circuit with a voltage trimming is described in U.S. Pat. No. 7,956,679, which also uses paired transistor switches. Each switch is controlled by a trim bit of a binary trim code. A certain number of transistors can be switched depending on the particular trim code applied to the transistor gates. U.S. Pat. No. 6,317,069 describes a binary-weighted transistor array of MOS devices. The devices provide a switching function and are enabled at any given time depending on the value of an associated binary bit input.
Semiconductor technology has evolved to using sub-micron geometries down to 90 nm or less. This technology allows integration of complex functionality into smaller areas and further, the use of sub-micron devices provides faster switching times. However, because of their smaller transistor channel lengths, current continues to flow even in the standby (non-switching) state, due to sub-threshold conduction. This conduction leads to current leakage. For low voltage threshold (LVT) devices in particular, drain current is significant even when the device is not enabled. Significant drain current leads to significant current leakage. When LVT devices are used in trimming switches for an internal clock source (such as the relaxation oscillator described above), current leakage can seriously affect the accuracy of the clock.
Internal clock sources require a trimming code with a linear relation if the trimming is to be effective and accurate. Thus, it would be advantageous to have a trimming circuit that can linearly trim the frequency of an internal clock source.